It has been reported that the injection of thymic cell-free extracts can restore the immunological function of neonatally thymectomized mice to reject skin grafts. Data showing partial reconstitution of neonatally thymectomized mice by thymus grafts in a Millipore chamber has suggested that the thymus acts as an endocrine gland and elaborates a hormone into the blood circulation. [See D. Osoba, et al., Nature 199, 653 (1963)]. The present invention is directed to this novel polypeptide hormone, designated thymic factor polypeptide hormone, and a novel method for the isolation and purification of this medicinally useful polypeptide.